1. Field of the Invention
The present invention relates to an optical encoder which is designed to convert light generated from a light source into parallel light rays by using a collimator lens, thereby allowing the collimated light to radiate to a grating and also to the collimator lens therefor. In particular, the present invention relates to an optical encoder capable of obtaining a favorable interference output appropriately used in optical encoders such as a linear encoder, a linear gauge and an arc-shaped encoder (a rotary encoder) employing point light sources such as a light emitting diode (LED) and a laser diode (LD) used as a light source, and also to a collimator lens therefor.
2. Description of Related Art
As shown in FIG. 1, as optical encoders used in a linear encoder, a linear gauge, an arc-shaped encoder (a rotary encoder) or the like, known is a 3-grating reflective-type encoder which is provided with a light source portion 12 including a light source 14 disposed inside a detector 10 and a collimator lens 16 for converting light generated from the light source 14 into parallel light rays, a first grating 22 for diffracting light radiated from the collimator lens 16 disposed on a transparent substrate 20 inside the detector 10, a second grating 32 formed on a scale 30 which relatively moves the detector 10 toward a measurement axis direction, a third grating 26 disposed on the transparent substrate 20 which further diffracts light diffracted by the first grating 22 and the second grating 32 disposed inside the detector 10, and a light receiving portion 24 including a light receiving element 28 which receives light diffracted by the first to the third gratings 22, 32, 26 (refer to Japanese Published Unexamined Patent Application No. 2003-279383 (Patent Document 1)).
In recent years, a light-generating source used as the light source 14 has become small in size and close to an ideal point-light source, interference becomes excessively intense, and as shown in FIG. 2, not only ±first-order or low-order diffraction light but also ±second-order and ±third-order or high-diffraction lights are also entered, therefore, as shown in FIG. 3, a favorable interference output cannot be obtained, which is a problem. In the drawing, G denotes a gap between the first and the third gratings 22, 26 and the second grating 32.
As an invention similar to the present invention, the applicant proposed in Japanese Published Unexamined Patent Application No. 2005-257521 (Patent Document 2) (especially FIGS. 1, 12, 13) that a diffusion plate is provided between the collimator lens 16 and the first grating 22, and/or a light diffusion portion is provided on the surface of the light source 14. The diffusion plate and the light diffusion portion are for giving a greater tolerance to deviation from the design value of gap G to improve alignment properties and not for obtaining a favorable interference output by preventing the second or the third high-diffraction light.